System for indicating the presence and location of fire



June 4, 1935. H, c GRANT, JR 2,003,358

SYSTEM FOR INDICATING THE PRESENCE AND LOCATION OF FIRE Filed Nov. 21, 1929 5 Shets-Sheet 1 UDCICIIJDCIU UEIQDDDUCID UDUDDUCIUUDCHJUIJDUUDDDCIDDUUUDDDIJ IIIDCHJDDUUDD n n u u m D a Q n u R z D ""i a N i u 33 cogumu TE N5 POWER 21E SUPPLY HA. 3A.

awwmtoz aw W7 June 4, 1935. H. (2. GRANT, JR 53 SYSTEM FOR INDICATING THE PRESENCE AND LOCATION OF FIRE Fi1ed Nov. 21, 1929 5 Sheets-Sheet 2 TO FIFTY TWO MOTOR Fm 0 1 Lu M Q awwemtoz June 4, 1935. H c A JR 2,003,358

SYSTEM FOR INDICATING THE PRESENCE AND LOCATION OF FIRE Filed Nov. 21, 1929 5 Sheets-Sheet 5 June 4, 1935. H, Q GRANT, JR 2,003,358

SYSTEM FOR INDICATING THE PRESENCE AND LOCATION OF FIRE Filed Nov. 21, 1929 5 Sheets-Sheet 4 June 4, 1935. H. c. GRANT, JR I 2,003,358

SYSTEM FOR INDICATING THE PRESENCE AND LOCATION OF FIRE Filed Nov. 21, 1929 5 Sheets-Sheet 5 DDUDDDDDDDDDDUDEIDDDDDDDDDDUUDDDEIDDDDUDEID TENS COLUMN UNITS COLUMN fi L UZeZZGrd/LZJT INVENTOR ATTORNEY5 Patented June 4, 1935 UNITED STATES PATENT OFFICE SYSTEM FOR INDICATING THE PRESENCE AND LOCATION OF FIRE Application November 21, 1929, Serial No. 408,740

12 Claims.

The present invention relates to systems for detecting the presence of fire and embodies, more specifically, an improved detecting system which indicates the exact location of such fire.

Most of the existing fire detecting systems merely indicate the presence of a fire within the area protected by the fire detecting system. In thermostatic fire detecting systems employing a considerable number of open circuited thermostats which are bridged across two wires of an external circuit, and in which the functioning of a thermostat causes a signal to be displayed at a central station which indicates the number of the circuit to which the thermostat belongs, there has been effected a grouping of the individual thermostats to indicate, by a suitable signal, the general location of the firej In such improved construction, however, it' has been necessary for the fire watch attendant to inspect the various spaces protected by the indicated circuit in order to determine the particular space in which fire has..occu rred.

The enormous expense of providing a system in which the exact location of a fire is ascertained by employing separate circuits for each thermostat and space to be protected, such circuits including the central station, will be readily apparent. As an improvement over the available fire detecting systems, the present invention provides a means for indicating the exact thermostat, and space p1 otected thereby, at the central station without requiring a number of wires running to the central station equal to the number of spaces or thermostats employed. With this minimum of wiring, a maximum amount of protection is afforded and a detailed signal, or indication, is quickly given to locate the fire definitely.

An object of the invention, accordingly, is to provide a fire detecting system which indicates the exactiocafion of a fire within the space protected by the system? A further object of the invention is to provide a system of the above character in which a plurality of heat responsive mechanisms are employed, the system being of such character that the specific heat responsive device actuated by a fire may be quickly ascertained.

As the invention is described more fully, and the character thereof more readily understood, further objects thereof will be apparent, the same being too numerous to attempt a detailed enumeration thereof herein and reference will now be had to the accompanying drawings wherein:

Figure 1 is a wiring diagram showing schematically a fire detecting system constructed in accordance with the present invention.

Figure 2 is a wiring diagram showing a modified fire detecting system constructed in accordance with the present invention.

Figures 3 and 4 are diagrammatic views showing the system of Figure 1 enlarged.

Figure 5 is a diagrammatic View showing the right hand portion of the system of Figure 1 enlarged to complete, with Figures 3 and 4, the enlarged showing of the system.

Referring to the above drawings, a battery is indicated at BAT. having its positive terminal connected to a positive bus wire I which includes a battery supervising relay 2, a switch So having switch points 3 and 4, connections 5 and 6, switch Sm having switch points I and 8, and positive bus wire 9. A plurality of external circuits, in which thermostats or other suitable heat operated devices are connected, is used in connection with the system shown and described herein, but for the purposes of description and in the interest of clarity, only two such circuits have been shown. These circuits have been marked as A and B and one side of each of these circuits is connected to the positive bus wire. Since the circuits A and B are identical and are connected to similar relays and associated elements, only the connection for circuit B will be described herein, it being understood that the corresponding elements of circuit A, as well as other circuits, not specifically shown, function in the same manner as the elements of circuit B. The other side of each external circuit is connected to the negative terminal of the battery and the elements for so connecting these circuits include, in the case of circuit B, a wire it, a contact I1, a trouble alarm relay T1, a fire alarm relay M1, wire I8, wire l9, and negative bus wire 20, such negative bus wire being connected to the negative terminal of the battery.

It will thus be seen that a supervisory current flows through the external circuit, the trouble alarm relay T1 and the fire alarm relay M1. If a break should occur in the wire so that current through the trouble alarm relay T1 is interrupted, this relay will be deenergized and will close contacts 2| and 22, causing a trouble alarm lamp L1 to be lighted. Current flows through the trouble alarm lamp L1 from the positive bus wire 9 through the lamp, contact 2|, to connection 23, contact 24, and connection 25 to the negative bus wire 20. This light indicates trouble in the external circuit B. At the same time, the general trouble alarm bell T. B. is sounded since it is connected to the positive bus wire through wire l0, point 26, point 21, point 28, point 29, through the bell T. B. point 38, wire 32, points 33, 35, and 31, contact 22, contacts 38 and 39 of switch S1, point 23 through contact 24 and point 25, to negative wire 20. In order to clear the general trouble alarm bell so that it may be free to indicate trouble in other circuits, the switch S1 is thrown to the left to make contact at point 40, thus opening the circuit through the general trouble alarm bell and clearing the bell for the indication of trouble in the other circuits. The trouble alarm lamp L1 remains lighted until the trouble has been repaired, whereupon the trouble alarm relay T1 picks up its armature thus opening the contacts 2| and 22. The trouble alarm lamp L1 remains lighted, however, by reason of its connection from the positive wire 9 to the successive points 40, 39, 23, 24, and 25. In order to clear the trouble alarm lamp L1, the switch S1 is thrown to its normal position at the right, as viewed in the figure, to make contact at point 38.

The battery supervising relay 2 is in series with the circuit supplying current to the external circuits, and if the current from the battery should fail, the battery supervising relay 2 will be deenergized and close contacts 4| and 42, thus causing the battery supervising trouble alarm lamp L2 to be lighted and the general trouble alarm bell T. B. to be sounded. A switch S7 is provided to clear the general trouble alarm bell in the same manner as that described in connection with the trouble alarm relay T1.

Two fire alarm bells are provided, one supervised and one unsupervised. The supervised fire alarm bell is indicated at S. A., while the unsupervised bell is indicated at U. A. The supervised bell is in series with a relay 43'which is of such resistance as to prevent the ringing of the supervised fire alarm bell, at the same time permitting a supervisory current to flow. The supervisory connection is from the positive wire 9 to the successive points, 26, 21, and 28, through the supervised fire alarm bell to the point 44, through the winding of the relay 43 and connection 45 to the point 46 on the negative bus wire 20. If the supervised alarm bell should become inoperative, the supervising current will fail and the relay 43 will become deenergized, thus permitting the closing of its contacts at 41 and 48, thus lighting the supervised fire alarm bell trouble alarm lamp L3 and sounding the general trouble alarm bell. The switch S5 is provided for clearing the general trouble alarm bell in the same manner as that described in connection with the trouble alarm relay T1.

A fuse 22| will be found in one lead to the supervised fire alarm bell. The reason for installing this fuse is that the resistance of. the winding of the supervised fire alarm bell is ordinarily only a fraction of one percent of the resistance of the winding of the fire alarm bell supervising relay 43. If a. short circuit of the supervised fire alarm bell leads should occur, no trouble signal would be given because the short circuit of the winding of the supervised fire alarm bell would allow a slightly greater supervising current to fiow through the supervising relay, this increase in current not being, however, great enough to blow the fuse 22 I. If a fire signal should be given, at which time the high resistance of the supervised fire alarm bell supervising relay is cut out of the circuit, the short circuit of the fire alarm bell connecting wires would constitute a direct short circuit across the source of current and, therefore, the fuse 22| is placed in one lead to the supervised fire alarm bell to relieve, immediately, this short circuit condition which otherwise would seriously affeet the operation of. the other circuits of the system. The blowing out of this fuse places the system in a condition to render a delayed trouble signal in a manner to be described hereinafter.

A resistance 49 is connected in the external circuit B at the extreme end thereof and this resistance is of such value as to limit the current which flows through the trouble alarm relay T1 and the fire alarm relay M1 to a value below that necessary to cause the fire alarm relay M1 to pick up. If, however, by reason of a fire in one of the spaces protected on the circuit B, a thermostat should function and thus short circuit the resistance 49, sufiicient current will flow through the circuit already described and cause the fire alarm relay M1 to pick up its armature and close contacts at points 50, 5|, 52, and 53, at the same time breaking the contacts at the points I! and 24. The fire alarm relay M1 is so constructed that the contact 5| will make before the contact I! breaks. The purpose of contact I! is to connect the resistance of the external circuit across one portion of a Wheatstone bri dgg asdescr-ibedfan-w hereinafter.

The opening of the contact I! would result in the deenergization of the fire alarm relay M1 were it not for the closing of contact 5| before the opening of contact l'l. By reason of. the closing of the contact 5|, a connection is made through the winding of the fire alarm relay M1 from the positive bus wire 9 to the successive points 54, 55, 56, 51, wire 58, connection 59, wire 69, connection 6| through contact 5|, the winding of fire alarm relay M1, wires l8 and I9, to the negative bus wire 29. The fire alarm relay M1, therefore, remains energized until the circuit through its winding is manually opened in the manner described hereinafter.

Upon the closing of contact 52, a relay 62 is energized by reason of its connection to the positive bus wide 9 through wire i9, and connection 26, cabinet door switch C. D. S. and to the negative line through point 63, wire 64, through the contact 52, to the point 25. Upon energization of the relay 62, contacts 66, 61 and 68 are closed, while contact 69 is broken. The closing of the contact 68 short circuits the supervised fire alarm bell supervising relay 43 and permits the supervised fire alarm bell to sound by reason of its connection from the positive Wire to the successive points 26, 21, and 28 through a wire 10 to the point 44, through a wire H, through contact 68, to the point 45, and thence to the point 46 on the negative line. By reason of the short circuiting of the supervised fire alarm bell supervising relay 43, the relay 43 is deenergized and the contacts 41 and 48 are closed. This would cause trouble alarm lamp L3 to light and the general trouble alarm bell to sound if it were not for the fact that the contact 69 of the relay 62 opens the connection from the trouble alarm lamp and from the general trouble alarm bell to the negative line. It will be seen that the trouble alarm lamp circuit is from the positive wire 9 to the successive points 26, 21, 12 through the trouble alarm lamp and contact 41 to a point 13 (where the line from the general trouble alarm bell joins a common return 14), through such common return to the contact 69 and the nega tive line wire.

The unsupervised fire alarm bell U. A. is caused to ring at the same time as the supervised fire alarm bell, the circuit for accomplishing such ringing being closed through the contact 61. Current flows from the positive wire 9 to the successive points 26, 21, 28, and 29, through the unsupervised fire alarm bell U. A. and through contact 61 to the negative line. Closing of the contact 66 produces an operation which will be described more fully hereinafter.

External circuit fire alarm lamps A, B, C, D, E, F, G, H, I, J', K, etc. corresponding to the external circuits A, B, etc. are connected in series with a supervising relay 15. The supervising current flows through a circuit from positive wire 9 through a wire III to a connection 16, through a wire I26 to a connection 11, through contact point 18 to connections 19, 80, wire BI, connections 82, and 83, contact point 84, connections 85 and 86, wire 81, connection 88, contact 89, connection 90 and wire 9I, connections 92 and 93, contact 94, connections 95 and 96, wire 91, connection 98, contact 99, connection I00, wire IOI, connections I02 and. I03, wire I04, relay 15, connection I05 and wire I06 to negative wire 20. Since the external circuit fire alarm lamps are in series, if any of the lamps become defective, the supervising current will cease and the relay will be deenergized, closing its contacts I99 and H0. Closing contact I09 establishes a connection from the positive wire 9 through wire III to point 16, through the trouble alarm lamp L4 through the contact I 09, point II 2, through wire H3 and contact II4 to wire H5 and the negative wire 20. At the same time, connection is established from the positive wire 9 to the successive points 26, 21, 28, and 29 through the general trouble alarm bell to the point through the wire 32 and point 33, wire II6 to contact II 0 and point II1, through switch S10 to point H2 and negative wire as described above, thus causing the sounding of the general trouble alarm bell. The switch S10 is provided for clearing the general trouble alarm bell in the manner already described in connection with the trouble alarm relay T1.

At the same time that a fire alarm relay, for example, M1 is energized with consequent energization of the relay 62, a relay H8 is energized by reason of its connection from the positive wire through the wire III to the points 16 and 11, through the winding of the relay II8 to a point H9 and through a wire I29 to the point 63 which is connected to the negative line through wire 64, contact 52 and point 25. Upon energization of the relay II 8, the contacts at 18, 84, 89, 94, 99 and H4 are broken, while contacts are closed at I2I, I22, I23, I24, and I25. The external circuit fire alarm lamp B, corresponding to the external circuit B, is then lighted by reason of its connection from the positive wire 9 through the wire III to the point 16, through wire I26 to point 11, through contact I2I to point 80, through wire 8| to point 82, through contact I22 to point I21, through con tact I23 to point I28, through contact I24 to point I29, through contact I25 to point I92, through wire IN and fire alarm lamp B to point I00, through wire I3I to contact 53, and to connection 25 on the negative line. The reason that the deenergization of the relay 15 does not result in lighting the trouble alarm lamp L4 and ringing the general trouble alarm bell upon the breaking of the contacts at 18, 84, 89, 94, 99, and H4 is that the contact H4 is in the line which connects the trouble alarm lamp L4 and the general trouble alarm bell to the negative line. When the contact I I4 is opened, the trouble alarm lamp L4 and the general trouble alarm bell cannot function.

The foregoing description has dealt chiefly with the various means for indicating trouble and with the means for indicating the particular circuit from which a fire signal has been given. It will be seen that the foregoing construction will function equally well regardless of the number of external circuits and the corresponding fire alarm lamps which are connected therewith. The following description is concerned principally with the means for detecting the exact thermostatic circuit closer which has functioned in the particular external circuit.

A Wheatstone bridge connection is provided including resistance elements I33 and I34, the resistance of the external circuit which has been actuated, and a variable known resistance indicated generally at I31. Point I32 of this connection corresponds to the connection of the resistance elements to the negative side of a battery circuit, while point I36 is the connection to a galvanometer and the unknown resistance which is the resistance of the external circuit. Point I is the connection to the other side of the galvanometer and to one terminal of the variable known resistance I31. It will be seen that point I32 is connected through a resistance I39 to points I39, I40, wire I4I, contact 249, contact 24I to the negative wire 20.

Point I35, on the other hand, is connected to the positive line through wire I44, resistance I45, movable contact I46, point I41, wire I48, resistance I49, and point I50 to a movable contact I5I and point I52 through wire I53 which is connected to the positive line through point I54 and wire' I55 to point 54 and positive wire 9. One side of the external circuit B is connected to the positive line and the other side of the circuit B, upon energization of the fire alarm relay M1, is connected to the point I39 through wire I6, contact 50, point I56, wire I51, point I58, wire I59 and point I36.

Upon the occurrence of fire in any space, the fire alarm relay, corresponding to the circuit by which such space is protected, is energized. A fire alarm 11m indicating the circuit is lighted and the fire alarm bellsargsounded. The resistance of that portion of the external circuit lying between the positive line and the particular thermostat which has functioned, is brought into proper relation with the Wheatstqr gg hri dgggrrangement for permitting e automatic balancing bT'the resistance of the external circuit and the consequent determination of the particular thermostat which has functioned. The variable resistance I31 consists of a rotary switch such as is now employed in automatic telephone work, resistances being connected between each of the adjacent points of the rotary switch.

With reference to the external circuit B, it will be seen that the path of the supervisory circuit from the positive wire 9 is through a compensating resistance I60, a resistance U1, the terminal of a thermostat TT1, a resistance U2, through the terminal of a thermostat TTz, etc. there being similar resistances before each thermostat. With reference to the variable resistance arrangement I 31, the resistance I49 is of the same value as the resistance U1; and the resistances R2, R3, R4, etc. are of the same values as the corresponding resistances U2, U3, U4, etc. all of the resistances being of the same value. If, therefore, the contact I46 is moved from the first point of the rotary switch to any other point, there can be cut into the Wheatstone bridge arrangement, enough resistance to balance the resistance from the external circuit which is thrown in as another leg of the Wheatstone bridge arrangement.

The automatic balancing of the resistance of the external circuit is accomplished through the medium of the Wheatstone bridge arrangement in combination with a galvanometer relay, the relay being so connected to the circuit that its contact remains closed as long as the balancing resistance I3! is less than the resistance of the external circuit. It will be seen that the variable resistance I31 has been shown with the movable contact I46 on the first point of the rotary switch. This is the normal position of rest at which time only the resistance I49 is in the balancing resistance circuit so that the resistance is always less than the resistance thrown in from the external circuit (except for the first resistance, which will be discussed later on). If, therefore, any thermostat beyond the first one functions, a resistance greater than the balancing resistance will be thrown across the proper leg of the Wheatstone bridge, and the galvanometer relay contact I 6I will be closed.

As soon therefore, as a fire alarm relay, such as M1 is energized, a relay I62 is energized by reason of its connection from the positive wire 9 through the connection 54, line I55, connection I54, wire I53, connection I63, wire I64, contact I65, wire I66, through relay I62, galvanometer contact I6I, wire I4I, contact 240, contact 24I to the negative wire 20. When the relay I62 is energized, the contact III is closed. This results in energizing relays indicated as Mot. I and Mot. 2, by reason of their connection with the positive wire I53 through the motor relay windings, the parallel relay contacts 243 and 244, respectively, wires 245' and 245" and connection 245, wire 246, connection 241, contact III, connection I 85 and wire I06 to the negative line wire 20. Upon the closing of the contact III of the relay I62, connection is established from the positive line to the negative line. The slow acting relay 231 is thus energized and contact I65 opened, thus deenergizing relay I62 so that contact I1] is opened and slow acting relay 23'! deenergized. This cycle of operation is repeated as long as the galvanometer contact I6I is closed. At the same time that the slow acting relay is deenergized by the opening of the contact I'II of the relay I62, the motor relays Mots. I and 2 are deenergized and simultaneously with the deenergization of the motor relays, the movable arm I46 moves one step from the first to the second point of the rotary switch. The same cycle is repeated until the amount of the balancing resistance I31 which has been cut in, is equal to, or greater than, or evensomewhat less than, the cut-in portion of the resistance of the external circuit. As soon as this condition is reached, the current flow through the galvanometer winding either weakens sufiiciently, ceases, or reverses, so that the galvanometer contact IGI is broken, as well as the circuit through the relay I62. The rotary switch has therefore come to a position of rest and can be made to indicate the exact thermostat which has functioned. This will be described in greater detail hereinafter. While in a theoretically perfect circuit the balancing resistance can be considered as exactly balancing the'resistance of the external circuit, the resistance of the external circuit might be somewhat less than the proper balancing resistance due to slight negative errors in winding the resistance units U1, U2, etc., while it might be somewhat more than the proper balancing resistance due to more wiring being used in the external circuit than the theoretically correct amount. The galvanometer relay is therefore adjusted to operate over a range such that the correct balancing resistance will be cut in not only when the resistance of the external circuit equals the proper balancing resistance, but also when it is within predetermined negative and positive variations from the proper balancing resistance.

Under certain conditions it might happen, that due to an excessive length of wire in an external circuit, the total resistance of the external circuit at the last thermostat, for example, consisting of the wire resistance of the circuit plus the total resistance of all of the unit resistances U1, U2, etc., will exceed the total balancing resistance sufficiently to make a balance impossible when the last thermostat functions, so that the rotary switches will continue to rotate in attempting to seek a balance. In order to make this continuous rotation impossible, it is desirable to maintain an open circuit (with infinite resistance) between points of the rotary switch, as shown in the drawings between the points 49th and 50th, so that a complete over-balance will be obtained when the last thermostat functions, thus causing the rotary switch to come to rest on the last point.

In the upper right hand corner of the figure there will be seen six rows of contacts. While all of these contacts could be arranged on a single rotary switch, it so happens that the rotary switches which are employed as standards for automatic telephone work contain the equivalent of three rows of contacts and it has therefore been necessary to illustrate connections to two rotary switches, which are operated in unison by the parallel motor relays I and 2. While any number of thermostats can be connected to a single circuit, the rotary switches diagrammatically indicated show contacts sufficient for the connection of fifty thermostats, this limitation being imposed by the commercially obtainable switches. Although it has been stated that a resistance is connected between each adjacent contact point of the switch, the fact is that these resistances are connected only between the first nine contact points, and one before the first, mak ing a total of nine resistances; the first, eleventh, twenty-first, thirty-first, and forty-first, being connected in parallel, similar connections being made for the other points. These connections are all made between the points of the first row of contacts. With reference to the second row of points, a resistance R10, equal to the value of the sum of ten of the unit resistances, is bridged across the points 9 and IE), points I9 and 28, points 29 and 30, and points 39 and 48. It will therefore be seen that no matter what the resistance is which is thrown in from the external circuit, the proper balancing resistance can be obtained through the movable contact I46, wires I48 or I36, and the movable contact IEI.

The third and sixth rows of contacts are employed in connection with the resetting of the system and will be described in detail hereinafter.

The fourth and fifth rows of points are provided for lighting suitable lamps to indicate the exact thermostat which has functioned. On the assumption that the first thermostat has functioned, the lamp of the tens column and the lamp I of the units column will be lighted in the following manner. The positive line wire 9 is connected to all of the lamps of each column and current flows through the proper lamp, in the hypothetical instance the lamp 6, of the tens column to point I'I4, through a movable contact I'I5, connection I76, wire H1, and wire I43 to the connection H9, wire I26 and connection 63 to wire 64, and contact 52 to connection 25 on the negative line. At the same time, current flows from the positive wire 9 through the units lamp I, wire I I9, movable contact I86, and through the connection I76 and path just traced. The connection of the points in the fifth row is such that a single set of lamps will serve for the unit indications, while the connection in the fourth row is such that one lamp is required for each of the tens of the series of thermostats and balancing resistances.

It will be understood that a Wheatstone bridge circuit will distinguish a definite resistance value most readily when the total resistance in the bridge circuit is low. The galvanometer relay must, therefore, be permanently adjusted to distinguish between the last and the next to the last thermostat (when the bridge circuit is highest) with the maximum available voltage across the bridge. If the bridge circuit resistance is now called upon to distinguish between the first and the second thermostat (Where the bridge circuit is lowest) it will be found so sensitive as to be affected by any low extra resistance inadvertently introduced by the circuit connecting wires, with the result that the rotary switch may advance one step beyond the correct point. In order to reduce this sensitivity at the low resistance end, the voltage across the bridge circuit proper is materially reduced by the introduction of a resistance, such as I38, placed in the voltage lead to the bridge. Therefore, in normal rest position, this resistance is in the bridge circuit so that regardless of the particular thermostat which may function, the bridge circuit first begins operation with its most insensitive adjustment. If a thermostat toward the end of the circuit has functioned, the bridge circuit automatically increases its sensitivity as it approaches the resistance ranges where increased sensitivity is needed. A sensitivity changer is shown as a relay I82. Assuming, by way of example only, that the resistance I38 is to be cut out after a balancing resistance equivalent to the external circuit resistance up to and including the tenth thermostat has been cut in, the short circuiting of the resistance I38 is accomplished by a connection from the positive wire 9 to the point I8I, through the sensitivity changer relay I82, contact I83, wire I84 and point I85 which is the negative side of the lamp I of the tens column, this lamp being the one that is lighted on the assumption already made that resistance I38 is to be cut out at this point. The re ay I82 is therefore energized, thus breaking the contact I63 and establishing contacts at I86 and I81. The contact I86 maintains the circuit through the winding of the relay I82 closed from the positive point I8I, through the wire I88, to the point I69, through the interconnected points I99 and movable contact I9I, to the negative line 26. As can be readily seen, the closing of the contact I31 directly short circuits the resistance I38. This resistance remains short circuited until such time as the movable contact I95 is returned to the first point of the switch, wh ch is not connected to the remaining points so that the connection from the point I99 to the movable contact I9I (now on the first point) is broken.

Relay 231 is a very slow acting relay to permit motors I and 2 to complete, with certainty, a cycle of operation before a new cycle is instituted. As before described, when galvanometer contact I BI is closed, connection is established from the positive line at I63, through contact I65, wire I66, relay I62 and galvanometer contact I6I to the negative line. Relay I62 is thus energized and contact I'II closed so that connection is established from the positive line at 228, through the winding of the relay 231, through contact "I and to the negative line. The relay 231 is thus energized and contact I65 opened thus deenergizing relay I62 so that contact I'II is opened and relay 23'I deenergized. This cycle of operation is repeated as long as the galvanometer contact I6I is closed. Each time that contact I'II is closed, connection is established from the positive line through the windings of the motor relays I and 2 through parallel contacts 243 and 244, connection 245, wire 246, to the point 24! and through contact I'II to the negative line, thus energizing Mots. I and 2. As already explained relay 231 is very slow acting and thus serves to insure complete operation of both motor relays I and 2 before relay 23'! opens the contact at I65. Such opening results in the deenergization of relay I62 and the opening of contact III, which results in the deenergization of the motor relays I and 2 and of the slow acting relay 231.

In order to reset the system, the push button switch I94 is operated to make contact with the point I92. Breaking the contact at 55 causes the deenergization of the fire alarm relay M1 with the consequent return to normal conditions of the apparatus controlled thereby. Connection is thus established from the positive line at 54 to the point I 92, through wire 250 to connection 25I, through the windings of relay 252 and 253, to points 254 and 255, to the interconnected points of the two rotary switches through wires 254' and 255'. From these respective interconnected points, the circuit is completed to the negative line through points 235 and 236. The relays 252 and 253 are therefore energized, causing the respective contacts at 24I and 243 for the relay 252, and 246 and 244 for the relay 253 to be broken and contacts 266, and 251, to be closed for the relay 253, while the corresponding contacts are closed for the relay 252. With the closing of the contacts 256 and 251, connection is established from the positive line I53, through points I52 and 232, through contact 251 and line 256, to the point 25I, through the windings of the relays 252 and 253 to the points 254 and 255 to the interconnected points of the rotary switches and thus to the negative line. Even though the push button switch I94 is returned to its normal position in contact with point 55, the relays 252 and 253 remain energized until such time as movable contacts I9I and I98 rest upon the first points of the respective rotary switches. Through the closing of the contact 266 of the relay 253, connection is established from the positive line through the motor relay 2, through the contact 266, contact I69, to the interconnected points of the rotary switch and thus to the negative line. Corresponding action occurs for relay 252 and results in energization of the motor relay I. At the same time, contact I69 is opened and the motor relay 2 is deenergized, causing the advance of the rotary switch one step. After such ad- Vance, the contact I69 is again closed and the motor relay is again energized. Such operation continues until the movable contact I9I rests upon the first point of the rotary switch. If, for any reason, motor 2 should get out of step with motor I, motor 2 will not continue to operate its rotary switch but will come to rest because it is no longer energized after the movable contact I9I rests upon the first point of the rotary switch. Motor I is therefore free to return the other rotary switch to its initial position without being affected by the action of motor 2.

The switch 208 is a push button switch similar to the push button switch I94 and interconnected with the switch I94 in a series parallel arrangement, and is controlled by the opening of the door of the cabinet in which all of the apparatus is arranged. Upon the occurrence of fire. the fire alarm bells sound and the system automatically balances itself. The fire watch attendant then runs to the central station and opens the door of the cabinet, thus causing the switch 208 to momentarily close the circuit from the point 56 to the point I93, causing a resetting action to occur in the same manner as caused by the push button switch I94. As soon, however, as the movable contacts reach the first switch points, the fire indication is repeated and the fire watch attendant can determine from the lamps which are lighted, the space which is afire. He can then push down the manual reset push button I94 and reset the system, and when the movable contacts again reach the first switch points the fire indication will be again given as a check upon the initial reading. This resetting operation can be made as many times as the fire watch attendant desires in order to be sure that the apparatus has functioned properly and that the proper space has been indicated.

After the fire has been extinguished and. the thermostat has been renewed, the door of the cabinet can be closed and the system will again be in its normal condition. If it should be forgotten to reset the system by means of the manual push button I94, the closing of the door will accomplish the resetting action through the closing of the push button switch 208.

In the wiring up of the external circuits, the distance between the various spaces protected, and consequently the amount of wiring between the adjacent thermostats, is most likely to vary and cause a variation in the total resistance of the circuit. It may be necessary, therefore, to install at certain predetermined intervals, compensating resistances, as for example, I60 and 209. These resistances are normally adjusted to an intermediate value and can be adjusted one way or the other to make the resistance at various points along the line the correct value, the value of the compensating resistance being such that it can balance out any variations which are likely to occur between itself and the preceding compensating resistance. A compensating resistance (for circuit B the compensating resistance is I60) is also required before the first thermostat due to the fact that there may be considerable variance in the resistance of the wires run from various points to the central fire detecting station. For every compensating resistance installed in an external circuit there must be a corresponding compensating resistance inserted between the proper contacts of the rotary switch. Thus compensating resistance I is inserted before the movable contact I46 to correspond to the compensating resistance I in the external circuit B. and. a

compensating resistance 2I0 is inserted between the fourteenth and fifteenth contacts in the second row of contacts to correspond to the compensating resistance 299 between the fourteenth and fifteenth thermostats in the external circuit B. Furthermore, it should be noted that whenever a compensating resistance is placed in any external circuit, a compensating resistance must be inserted in the corresponding position in every other external circuit.

A door switch C.D.S. is provided in the circuit of the fire alarm bell ringing relay 62, so that upon the opening of the cabinet door the fire alarm bells will stop sounding. If, however, the cabinet door is closed before the short circuited thermostat has been restored with a new, open circuited thermostat, the bell will again ring and indicate the failure .to restore the thermostat. Attempting to reset the apparatus without restoring the thermostat will merely result in the repetition of the fire signal.

If on the other hand, the cabinet door is left open, as it should be until the thermostat has been restored and if the thermostat is removed from the line (the thermostats are not bridged across two continuous wires, but the external circuit wires are run in sections between the thermostats, and the thermostats are designed so that when removed, they break both sides of the external circuit and thus initiate a trouble signal), the trouble signal will not be given because the fire alarm relay has remained energized and the trouble circuit has been disconnected, but the wire break left by the removal of the thermostat constitutes an infinite resistance thrown across one leg of the Wheatstone bridge, and the arm of the rotary switch advances to the last point of the balancing arrangement when an infinite resist ance is then thrown in to balance the infinite resistance of the open external circuit. When the thermostat has been restored, the arm of the rotary switch still rests on the last point because the current limiting resistance in the external circuit is great enough to require the infinite resistance to balance it, no other balancing resistance being available. All during this time the external circuit fire alarm lamp has remained lighted. If now the cabinet door is closed, the reset button operated by the closing action of the door will cause the fire alarm relay to be deenergized and the system to be restored to its normal condition, the rotary switch arm advancing to the first point.

If on the other hand, someone should close the cabinet door between the time the short circuited thermostat was removed and a new thermostat installed, the fire alarm signals will be cleared immediately, and the trouble signal will be given. If, still further, a thermostat should be removed from an external circuit, the condition of the system being initially normal (for example, no fire signal has been previously given), the trouble signal will be immediately given, indicating a wire break.

Reference has been made hereinbefore to the use of a fuse 22I in the supervising circuit of the supervised fire alarm bell for the purpose of preventing the failure of the system due to a short circuit of the supervised fire alarm bell. As previously described, the fuse 22I is designed to blow out at the time that a fire signal is given. As soon as the cabinet door is opened to stop the sounding of the fire'alarm bells, the bell ringing relay 62 is deenergized and the circuit through the trouble signal is closed so that the delayed trouble signal hereinbefore referred to is thus given, together with the fire signal.

It is contemplated that the thermostats TT1, TTz, etc., and the corresponding unit resistances U1, U2, etc, be built as complete working units. Under these circumstances it is conceivable that a fire might be so severe as to destroy the unit resistance of the functioning thermostat so that the fire signal would not be given except momentarily, after which an open circuit would exist and the rotary switch arms would come to rest on the last switch point (for the reason previously given), instead of indicating the space afire. In order to overcome this difficulty, the thermostats and unit resistances may be built as units as before, but the resistance associated with the first thermostat is to be connected into the lead to the second thermostat instead of into the lead from the central station to the first thermostat. The unit resistance associated with the first thermostat will control the fire signal given by the second thermostat, the second and first unit resistances will control the fire signal given by the third thermostat, etc., so that the destruction of a unit resistance associated with a particular thermostat will not effect the fire signal to be given by that thermostat.

In the explanation given at the beginning of this description, the battery was pointed out as the source of energy for furnishing the supervising current. This reference was made for simplification of the description. In reality the battery is under trickle charge, the actual current normally supplying the supervising current to the positive line 9 being obtained directly from the power supply terminals indicated to the right of the battery terminal. The normal path of the supervising current is from the positive ter' minal 2i 2 of the power supply to the point 2|3 through the contact 2M through the successive points 215, 2H5, and 2!], through the wire 218, through the contact 2l9, through the successive points 5, 6, 1, and 8 and then through the external circuits back to the negative line in the manner already described. It will be noted that the contact 2H9 short circuits the resistance 22!).

The path or" the battery charging current is from the positive terminal 2l2 of the power supply to and through the successive points 2l3, 21A,

2 l5, 2H5, and 2 ll, through the contact M9, to and through the successive points 5, 4, and 3, through the winding of the relay 2, to the positive terminal l of the battery, and then to the negative line.

It so happens that when the fire alarm bells are sounding, a much greater current is required from the power supply, and the tendency would be for the voltage across the system to decrease below the battery voltage. In order to prevent such decrease, a portion of the resistance in the power supply circuit is cut out. This resistance is shown at 223 and is short circuited by the closing of the contact 66 of the bell ringing relay 62.

On the other hand, if the battery should for any reason go off the line, a smaller current would be drawn from the power supply, and the tendency would be for the voltage across the system to increase. In order to prevent such increase, additional resistance must be added to the power supply line. This resistance is shown at 228, and.

is added by the removal of the short circuit at the contact 2 l 9 of the battery supervising relay 2, which is deenergized upon the failure of the battery, with the result that the contact 2|9 is opened.

The power supply is supervised by the relay 224 which is deenergized upon the failure of the power supply, causing the contacts 225 and 226 to be closed, at the same time opening contact at 2| 4 in order to prevent the discharge of the battery back through the circuits which may also be connected across the power supply. The closing of the contact 225 results in the lighting of the trouble alarm lamp L5 and the closing of the contact 226 results in the sounding of the general trouble alarm bell. The switch S8 is provided to clear the general trouble alarm bell, in the manner already described in connection with the trouble relay T1.

The switches So and SM and the current meter M are provided for determining the various currents supplied by the source. With the switch SM in the position shown, the meter is short circuited. If the switch SM is thrown to the left, the path of the current through the meter will be from the positive terminal 212 of the power supply to the point 2|3, through the contact 214, to and through the successive points 215, 2I6, 2H, through the wire 2H3, through the contact M9 to the point 5, to the point 6, through the meter M to the point 285 to the positive line 9, through the external circuits to the negative line, so that the supervising current is read by the meter. If, in addition to throwing the switch SM to the left, the switch S is thrown to the left, the charging current of the battery also passes through the meter so that the meter reads the sum of the supervising and charging currents. The connections for passing the battery charging current through the meter are from the positive terminal 2|2 of the power supply to the point 2l3, through the contact 2M, to and through the successive points 2l5, H6, 2, and 6, through the meter M, to the point 285 to the point 222 to the point 3, through the winding of the relay 2 through the battery to the negative line.

It will be seen that the above circuit could be greatly simplified if assurance could be made that the motor relays l and 2 would always have exactly the same characteristics. Since, however, one of such relays is likely to operate somewhat faster than the other and thus get out of step, the foregoing construction overcomes any possibility of this happening. Without such a construction the motor relays, if once out of step, would cause the rotary switches to rotate indefinitely.

The foregoing description has dealt with two rotary switches. The invention, however, is capable of working with a single rotary switch of proper design and Figure 2 shows a construction in which a single motor 26l is used for the single rotary switch. In this figure, connections to the positive line are shown at 262 and 263, while connections to the negative line are shown at 264, 265, 266, 261, and 268. In the normal operation of the system, when a fire signal is given, the contact at I 6| on the galvanometer relay is closed and the connection is established from the positive line at 252 through the contact 269 to the point 218 through the winding of the relay 2' through the galvanom-eter contact l6l, through the contact 212 to the negative line at 261. The relay 2' is therefore energized and the contact 213 is closed. Connection is then established from the positive line at 262 through the contact 214, through the winding of the relay 26| to the point 215 through the contact 213 to the negative line at 264. When the motor 26! is energized the breaking of the contact at 269 results in the relay 2H being deenergized so that contact213is opened and the motor 26! is deenergize-d, at which time the rotary switch arm is advanced one step. The same cycle of operation is repeated until a proper balance is reached which indicates exactly the location of the fire.

In order to reset the apparatus, the push button 216 is pressed to make contact at the point 2?! (the breaking of the contact at 286 causes the deenergization of the fire alarm relay M1 with consequent return to normal conditions of the apparatus controlled thereby), so that connection is established from the positive line at 263 to the point 218 to the point 211, through the win-ding of the relay 219 to the point 280 to the interconnected points of the rotary switch and through the movable contact 28| to the negative line at 266. The relay 219 is energized, with the result that the contacts at 212 and 214 are broken, while contact is made at the points 282, 283, and 284. With the closing of the contact at 282, connection is established from the positive line at 263 to the point 218, through the contact 282, through the winding of the relay 219, to the point 280, to the interconnected points of the switch through the movable contact 28!, to the negative line at 266. Even though the push button switch 216 has returned to its original position, the relay 219 remains energized until the movable contact 28I rests upon the first point of the switch. Through the closing of the contacts 283 and 284, connection is established from the positive line at 262 through the contact 269 to the point 210 through the contact 284 through the winding of the motor 26I to the point 215 through the contact 283 to the point 280 to the interconnected points of the rotary switch through the movable contact 28| to the negative line at 266. With the energization of the motor 26L the contact 269 is opened so that the motor is deenergized at which time the rotary switch arm advances one step, after which time the contact 269 is again closed, and the cycle of operation is repeated until the movable contact 28! rests upon the first point of the switch.

With the conections shown in Figures 1 and 2, if it should be forgotten to restore the thermostat which has functioned, the operation of the reset push button will return the rotary switches to their initial position on the first points, but immediately the fire signal corresponding to the defective thermostat will be repeated. This thermostat must be restored before the system can be returned to its normal condition.

Throughout the above description it has been pointed out that the normal position of the rotary switch or switches, is on the first contact point. The reason that the switches return to this point is that the first point is the only dead point, and through this point the motor relays can receive no energy. If a fire should occur in any of the number I spaces of the various circu ts, the rotary switch or switches will remain on the first contact but in addition, the fire alarm lamp corresponding to the external circuit on which fire has occurred will light, the tens lamp 6 and the units lamp I will light, and the fire alarm bells will sound, thus indicating the existence and the location of the fire.

From the foregoing description, it will be apparent that a fire detecting system is provided embodying a minimum of wiring, the system being completely supervised against wire breaks,

power supply trouble, battery trouble, external circuit alarm trouble, and external circuit fire alarm lamp trouble. A single general trouble alarm bell may be employed which may be readily cleared at all times for the indication of trouble in other circuits. The trouble alarm lamps will remain lighted until the trouble has been repaired and until the trouble alarm lamp switches have been returned to their normal positions, and the system is capable of indicating exactly the location of a fire, either through manual balancing means, or automatic balancing means. The system provides for checking the results rendered either manually or automatically, and permits the checking of the restoration of the thermostat that has functioned since the system cannot be returned to its normal condition unless the thermostat is restored. Variations in the resistance of the etxernal circuits due to variations in wiring can be easily compensated for and the apparatus is capable of being arranged in a single cabinet with an appropriate door switch so that the sounding of the fire alarm bells can be stopped by the opening of the cabinet door and so that the fire must be attended to and the thermostat replaced before the cabinet door can be closed again without the repetition of the fire signal. In the event that the system is not properly reset, the closing of the cabinet door can be made to operate a resett ng switch.

While the invention has been described with specific reference to the accompanying drawings, it is not to be limited, save as defined in the appended claims.

I claim as my invention:

1. An electro-responsive system comprising a plurality of external circuits each having two circuit wires adapted to be mounted in areas to be protected, a plurality of resistances in each said circuit connected in series therein, a plurality of circuit controlling means bridged across the circuit wires of each said circuit between successive resistances of said series of resistances and adapted to be located within diiferent areas to be protected whereby each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, circuit indicating means for each circuit, area indicating means for each circuit controlling means including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the last named indicating means and one of the external circuits with certain of its resistances, means operable upon actuation of a circuit controlling means in an external circuit to connect said circuit as an element of the Wheatstone bridge and to actuate the respective circuit indicating means, and means actuated by the Wheatstone bridge to vary the area indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge for efiecting a proper area indication.

2. An electro-responsive system comprising a normally closed external circuit extending through areas to be protected including a plurality of resistances connected in series therein and adapted to have a supervisory current flow therethrough, a plurality of circuit controlling means adapted to be located within difierent areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, area indicating means for each circuit controlling means and including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the indicating means and the external circuit with certain of its resistances, means operable upon actuation of a circuit controlling means in the external circuit to connect the circuit as an element of the Wheatstone bridge, and means actuated by the Wheatstone bridge to vary the area indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge for efiecting a proper area indication.

3. An electro-responsive system comprising a plurality of external circuits, each including a plurality of resistances and extending through areas to be protected, a plurality of circuit controlling means for each said circuit adapted to be located within difierent areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, circuit indicating means for each circuit, area indicating means for each circuit controlling means including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the last named indicating means and a selected external circuit with certain of its resistances, means operable upon actuation of a circuit controlling means in an external circuit to connect said circuit as an element of the Wheatstone bridge and to actuate the respective circuit indicating means, and means actuated by the Wheatstone bridge to vary the area indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge for efiecting a proper areaindication.

4. An electro-responsive system comprising a plurality of normally closed external circuits, each extending through areas to be protected and including a plurality of resistances connected in series therein and adapted to have a supervisory current flow therethrough, a plurality of circuit controlling means for each said circuit located within different areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, circuit indicating means for each circuit, area indicating means for each circuit controlling means including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the last named indicating means and a selected external circuit with certain of its resist ances, means operable upon actuation of a circuit controlling means in an external circuit to connect said circuit as an element of the Wheatstone bridge, and means actuated by the Wheatstone bridge to vary the area indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge for efiecting a proper area indication.

5. An electro-responsive system comprising a plurality of normally closed external circuits each having an outgoing and a return wire and extending through areas to be protected, a plurality of resistances in each said circuit connected in series therein and adapted to have a supervisory current fiow therethrough, a plurality of circuit controlling means bridged across the outgoing and return wires of each said circuit between successive resistances of said series of resistances and adapted to be located within diiferent areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, circuit indicating means for each circuit, area indicating means for each circuit controlling means including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the last named indicating means and any one of the external circuits with certain of its resistances, means operable upon actuation of a circuit controlling means in an external circuit to connect said circuit as an element of the Wheatstone bridge, and means actuated by the Wheatstone bridge to vary the area indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge for effecting a proper area indication.

6. An electro-responsive system comprising a plurality of normally closed external circuits, each extending through areas to be protected and including a plurality of resistances connected in series therein and adapted to have a supervisory current flow therethrough, a plurality of circuit controlling means for each said circuit located within difierent areas to be protected, the resist ances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, circuit indicating means for each circuit, area indicating means for each circuit controlling means, a plurality of resistances corresponding to the first named resistances, connections between the last named resistances and the area indicating means, a Wheatstone bridge including the first and last named resistances as the variable elements thereof, means actuatable upon actuation of a circuit controlling means in an external circuit to connect said circuit with certain of its resistances as an element of the Wheatstone bridge and to actuate the respective circuit indicating means, and means actuated by the Wheatstone bridge to vary the second named resistances to effect a proper area indication.

7. An electro-responsive system comprising a plurality of normally closed external circuits, each extending through areas to be protected and. including a plurality of resistances connected in series therein and adapted to have a supervisory current fiow therethrough, a plurality of circuit controlling means for each said circuit located within different areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, circuit indicating means for each circuit, area indicating means for each circuit controlling means in a circuit, a plurality of resistances corresponding to the first named resistances, connections between the last named resistances and the area indicating means, a Wheatstone bridge including the first and last named resistances as the variable elements thereof, means actuable upon actuation of a circuit controlling means in an external circuit to connect said circuit with certain of its resistances as an element of the Wheatstone bridge, and means actuated by the Wheatstone bridge to vary the second named resistances to effect a proper area indication.

8. In combination with an electro-responsive circuit, a plurality of heat actuated devices connected in parallel in the circuit, a resistance element for each device having a partial controlling efiect on an indication to be given, means to mount the devices and resistances as units, each unit comprising one device and one resistance, circuit responsive means responsive to the actuation of any one of the devices and controlling the initiation of the indication to be given, each circuit controlling means serving to short circuit all resistances in the circuit more remote from the circuit responsive means, and connections to connect the resistance of one unit with the device of the unit next most remote electrically from the circuit responsive means for control by said device, whereby destruction by fire of a resistance mounted with a device will not alter the controlling eifect of said device and its controlled resistance, which latter is the resistance mounted with the device next nearest electrically to the circuit responsive means.

9. An electro-responsive system comprising an external circuit including a plurality of resistances and extending through areas to be protected, a plurality of circuit controlling means located within different areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, area indicating means for each circuit controlling means including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the indicating means and the external circuit with certain of its resistances, means operable upon actuation of a circuit controlling means in the external circuit to connect the circuit as an element of the Wheatstone bridge, means actuated by the Wheatstone bridge to vary the area indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge for effecting a proper area indication, and adjustable compensating resistance in the circuit to balance out variations from standard in the values of the various resistances and the resistance of the circuit.

10. An electro-responsive system comprising an external circuit including a plurality of resistances and extending through areas to be protected, a plurality of circuit controlling means located within different areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, area indicating means for each circuit controlling means including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the indicating means and the external circuit with certain of its resistances, means operable upon actuation of a circuit controlling means in the external circuit to connect the circuit as an element of the Wheatstone bridge, means actuated by the Wheatstone bridge to vary the area indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge for effecting a proper area indication, and adjustable compensating resistances located at suitable points in the circuit to balance out variations from standard in the values of the various resistances and the resistance of the circuit.

11. An electro-responsive system comprising an external circuit including a plurality of resistances and extending through areas to be protected, a plurality of circuit controlling means located within different areas to be protected, the resistances and circuit controlling means being connected in such fashion that each circuit controlling means serves to short circuit the resistances in the respective circuit more remote from the circuit responsive means, area indicating means for each circuit controlling means including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the indicating means and the external circuit and certain of its resistances, a sensitivity changing resistance normally in series with the Wheatstone bridge, means operable upon actuation of a circuit controlling means in the external circuit to connect the circuit as an element of the Wheatstone bridge, means actuated by the Wheatstone bridge to vary the indicating means and to balance the second named resistances against the first named resistances included in the Wheatstone bridge, and means to short circuit the sensitivity changing resistance to change the voltage on the Wheatstone bridge after a predetermined actuation of the indicating means.

12. An electro-responsive system comprising a plurality of similarly actuatable resistance changing devices and resistances located at different points but connected in a single circuit through which a supervisory current is adapted normally to flow, indicating means for each resistance changing device in the circuit and including a plurality of resistance elements corresponding to the first named resistances, a Wheatstone bridge including the indicating means and the external circuit with certain of its resistances, switching means responsive to a change in value of the supervisory current due to actuation of one of the devices for connecting the circuit to the Wheatstone bridge to vary the indicating means and to balance the bridge with the second named resistances for indicating the actuated device.

HARRY CAMPBELL GRANT, JR. 

